In the past, it has been known to manufacture so-called "printed" circuits by various processes where droplets of conductive metal are sprayed onto an insulating substrate through a mask or stencil to define conductive patterns on the substrate, rather along the lines of a spray-painting technique. Two examples of such prior-known processes are disclosed in A. M. Hathaway U.S. Pat. No. 2,599,710 and T. E. Fairbairn U.S. Pat. No. 3,607,381 both herein incorporated by reference.
Various techniques and metal "flame-spraying" equipment have become available on the market, which render this basic process attractive for the mass production of very low-cost, relatively simple printed circuts, particularly for low-power, digital logic applications such as electronic keyboards for pushbutton telephone "dials," where the voltage and current requirements are low and where extremely fine control over electrical resistance of conductive patterns and extremely fine line widths, etc. are not required.
Specific objects of the present invention are to provide improvements in the prior-known flame-spraying process for making printed circuits of the type described above, particularly improvements with respect to combinations of materials for such circuits and the stencil, and to new and improved conductive through-hole constructions for two-sided circuit boards of this type.
More general objects are to provide improved low-cost printed circuit boards and manufacturing techniques, particularly for digital circuits, and particularly to provision of printed circuits and contact pads having excellent wear characteristics.